Sealing devices that seal an annular gap between a housing and a shaft have hitherto been known. In some known sealing devices used in equipment where the shaft largely deviates relative to the housing, a sealing part is provided on a radially inner side of a thin bellows part so that the sealing part in contact with a shaft can follow the shaft as the shaft deviates (see PTL 1).
A sealing device according to such a prior art example will be described with reference to FIG. 7. FIG. 7 is part of a schematic cross-sectional view of the sealing device according to the prior art example. The sealing device 500 according to the prior art example shown in FIG. 7 includes a fixed part 510 fixed to an inner circumference of a shaft hole in a housing, a thin bellows part 520 extending from the fixed part 510 toward a shaft surface in a bellows shape, and a sealing part 530 provided on a radially inner side of the bellows part 520. The sealing part 530 makes slidable contact with an outer circumferential surface of the shaft. In the sealing device 500 according to this prior art example, when the shaft deviates, the sealing part 530 stays in tight contact with the shaft, while the bellows part 520 undergoes deformation to keep the sealing part 530 consistently in tight contact with the outer circumferential surface of the shaft, thereby to maintain the airtightness.
Nevertheless, there is an issue that the bellows part 520 may suffer cracks or splits because the shaft repeatedly deviates at the same time as it rotates. This will be explained with reference to FIG. 8. FIG. 8 is a schematic diagram of the sealing device 500 viewed from an axial direction when the shaft deviates as it rotates. In FIG. 8, reference numeral 500P indicates the position of an inner peripheral edge of the sealing part 520, while 500Q indicates an outer peripheral edge of the fixed part 510.
When the shaft deviates during rotation, the inner peripheral edge 500P of the sealing part 520 moves in a direction (direction of arrow A) in which it deviates from the outer peripheral edge 500Q of the fixed part 510 with the movement of the shaft in the deviating direction. At the same time, the inner peripheral edge is subjected to a force (torque) in a rotating direction (direction of arrow B) as the shaft rotates. The bellows part 520 is twisted because of this, and stress concentrates in some part thereof (part X in the drawing). Therefore, as the shaft repeatedly deviates and rotates at the same time, the stress concentration is repeated, which could result in cracks or splits. The partial twisting could also deteriorate the ability of the sealing part 520 to follow the shaft, or make the orientation of the sealing part 520 unstable, which may reduce the airtightness.
As described above, one of the important issues in sealing devices that have a thin bellows part is how to mitigate stress concentration in the bellows part.